Dual chamber heart stimulator with evoked response detector

ABSTRACT

A heart stimulator has an atrial and ventricular pulse generator for producing atrial and ventricular stimulation pulses, an atrial sensor for sensing atrial signals and an evoked response detector for detecting the occurrence of incipient fusion beats from measured ventricular signals. A determination unit determines an incipient fusion AV-interval from the sensed atrial signals and the detected fusion beats, and a controller controls the pulse generator to deliver stimulation pulses at a controlled AV-interval which is shorter than the incipient fusion AV-interval. The evoked response detector includes an averaging unit which forms an average amplitude value of the measured ventricular signals during a predetermined time window of each cardiac cycle, and a comparator which compares the average value for each cardiac cycle with a predetermined limit criterion, and supplies the result of the comparison to the determination unit for determining a measured ventricular signal resulted from an incipient fusion beat or a completely stimulated capture.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a heart stimulator, particularly a dualchamber pacemaker with an evoked response detector.

2. Description of the Prior Art

For certain conditions such as hypertrophic obstructive cardiomyopathy(HOCM) the patient's condition may improve if he or she is paced 100% inthe ventricle. In a state of HOCM the left ventricular wall isasymmetrically thickened. The interventricular septum thicknesssignificantly exceeds that of the opposing posterolateral wall. Apressure gradient exists across the left ventricular outflow tract andduring ventricular contraction, a progressive degree of outflow tractobstruction results. The conventional site of ventricular pacing iswithin the right ventricular apex and pacing, prior to intrinsic R-waveexcitation, from this site can favorably alter the degree ofobstruction. This has been clinically verified.

100% pacing in the ventricle requires understanding of a phenomenonreferred to as fusion. Fusion means that the natural conduction time,which is the time interval between an atrial activity (a sensed P-waveor a delivered A-pulse) and the subsequent natural ventricular activity(R-wave), is the same as the time (AV-interval) between an atrialactivity (again, a sensed P-wave or a delivered A-pulse) and thedelivery of a ventricular stimulation pulse (V-pulse). Fusion is thus acondition where the V-pulse is delivered at the same time as the R-waveoccurs. Thus, fusion means that the V-pulse occurs when the heart tissueis not capable of responding, i.e. it is refractory, a tissue refractoryperiod starting at the depolarization event (R-wave) and remaining untilrepolarization (T-wave) occurs. Although not necessarily harmful to theheart, fusion causes loss of energy in the V-pulse, and should thereforebe avoided to save pacemaker battery energy. In the discussion herein,both the time interval between a P-wave or an A-pulse, and a V-pulsewill be referred to as the AV-interval.

To obtain 100% pacing beats with no fusion, very short AV delays havebeen used. Such very short AV intervals are, however, non-physiologicand therefore it is highly desirable to prolong the AV interval whilemaintaining a continuous monitoring of fusion, such that the AV intervalwould be shortened automatically if fusion beats appear. Severalattempts have been made to solve this problem.

Thus, U.S. Pat. Nos. 5,534,016 and 5,713,930 describe techniques foroptimizing the AV interval for therapeutic purposes for patients havingHOCM. In the system according to U.S. Pat. No. 5,534,016 the T-wavedetection is monitored to detect when the AV interval is lengthened tothe point of evoking a fusion beat, and in the system disclosed in U.S.Pat. No. 5,713,930 the relationship between AV intervals and OTintervals (=the time interval between a delivered ventricular stimulusand resulting T-wave) is monitored and therefrom it is determined whenAV intervals correspond to full capture and when AV intervals correspondto fusion.

Further, in U.S. Pat. No. 5,507,782 a dual chamber pacemaker isdescribed in which the longest AV interval which results in completeventricular capture is determined from the wave form of the ventriculardepolarization R-wave following a ventricular pacing pulse for thepurpose of treating patients suffering from HOCM. In this document theproblems related to fusion beats and the transition region betweencomplete pacing and fusion are not at all dealt with.

Another way of solving the problem of fusion and providing a 100% pacingof the ventricle is by AV node ablation. AV node ablation is, however,an intervention associated with extra costs and the conduction pathwayfrom the atria to the ventricles is then permanently destroyed so thepatient will be completely dependent on a pacemaker in the future withhigher clinical risks in the event of a pacemaker failure.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a heart stimulatorsuitable for treating HOCM patients by accomplishing 100% pacedventricular capture, which stimulator comprises a new type of evokedresponse detector suitable for detecting incipient fusion in a simpleand reliable way.

The above object is achieved in accordance with the principles of thepresent invention in a heart stimulator having an atrial stimulator anda ventricular stimulator for producing stimulation pulses respectivelyfor delivery to the atrium and the ventricle of a patient's heart, anatrial sensor for sensing atrial signals, an evoked response detectorfor detecting evoked response signals, a determination unit fordetermining an incipient fusion AV-interval from the sensed atrialsignals and the detected evoked response signals and a control unit forcontrolling the ventricular stimulator to deliver ventricularstimulation pulses at a controlled AV-interval which is shorter than theincipient fusion AV-interval, wherein the evoked response detectorincludes an averaging unit which forms an average amplitude value of theevoked response signal during a predetermined time window of eachcardiac cycle, and wherein a comparator compares this average value withpredetermined limit criteria and supplies a comparison result to thedetermination unit to allow the determination unit to determine whethera detected evoked response signal results from an incipient fusion beator a completely stimulated capture.

In the following the expression “sensed atrial signals” denotes sensedspontaneous atrial events P-waves as well as stimulated atrial events(A-pulses). The interval between a sensed spontaneous atrial event andthe ventricular V-pulse is denoted by PV interval, and the intervalbetween a stimulated atrial event and the ventricular V-pulse is denotedby AV interval. The PV interval is generally shorter than the AVinterval. As noted above, however, as used herein the PV-interval aswell as the AV-interval will be referred to as the AV-intervalhereinafter.

Thus, in the stimulator according to the present invention theAV-interval is continuously monitored and automatically shortened ifincipient fusion is detected. Incipient fusion is detected by an evokedresponse detector from measured ventricular signals picked up by anventricular electrode and containing the evoked response signal, and theAV-interval will be adjusted accordingly to be as long as possible whileavoiding the occurrence of fusion beats. From a hemodynamic point ofview, e.g. ventricular filling and cardiac output, such a stimulatoroperation will give optimum results. Thus the stimulator according tothe invention will operate with an AV-interval that is optimized withrespect to hemodynamic conditions.

In an embodiment of the heart stimulator according to the invention, thecontrol unit is adapted to modulate the AV-interval with a predeterminedamount, and the comparator is adapted to compare the variation of theaverage amplitude values obtained during the time window with apredetermined limit. A large variability is then a clear indication ofincipient fusion.

In another embodiment, the control unit is adapted to regularly prolongthe AV-interval with a predetermined amount and the comparator isadapted to compare the average amplitude values obtained during the timewindow of cardiac cycles with the predetermined limit value and/orcompare the variation of the average amplitude values obtained duringthe time window with a predetermined limit. In this way, incipientfusion can be detected at a very early stage and by utilizing both anamplitude criterion and a variability criterion improved reliability isobtained.

In a further embodiment of the heart stimulator according to theinvention the evoked response detector is adapted to determine the DClevel of the measured ventricular signal and subtract this DC level fromeach sample before the average value is formed. It is important tosubtract the DC level from, the measured signal picked up by theelectrode to get a corrected signal for subsequent analysis.

In another embodiment of the heart stimulator according to the inventiona respiration signal determining unit is provided for determining arespiration signal representing the respiration rate of the patient,from a predetermined number of the average values.

DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of the basic components of a heart stimulatorconstructed and operating in accordance with the principles of thepresent invention.

FIG. 2 is a block diagram of the evoked response detector of the heartstimulator according to the invention.

FIG. 3 is a flowchart illustrating the operation of the inventiveembodiment shown in FIGS. 1 and 2.

FIG. 4 is a block diagram of the basic components of a second embodimentof a heart stimulator constructed and operating in accordance with theprinciples of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

It is known to distinguish between completely stimulated captures,fusion beats and losses of capture from analysis of average amplitudevalues of recorded ventricular signals during a predetermined timewindow after a pacemaker stimulation, see Åsa Uhrenius et al.,“Evaluation of new Algorithms for Autocapture with Unipolar Leads”,CARDIOSTIM 98, Nice, June 1998.

FIG. 1 shows a block diagram of the basic components of the heartstimulator according to the invention. The stimulator has a pulsegenerator 2 which through leads 5, 6 and associated atrial andventricular electrodes 7, 9 are connected to the heart 8 of a patient.The pulse generator 2 is devised to produce stimulation pulses ofvarying amplitudes which through the leads 5, 6 with their electrodes 7,9 are transferred to the heart 8. An evoked response detector 4 of theabove mentioned type is connected to the ventricular lead 5. An atrialdetector having an atrial filtering and measuring unit 17 is connectedto the lead 6 for measuring amplitudes of signals picked up by theatrial electrode 9. A determination unit 13 is connected to the evokedresponse detector 4 and to the atrial filtering and measuring unit 17for determining an incipient fusion AV-interval, i.e. the AV-interval atwhich incipient fusion was detected, from the measured atrial signalsand detected incipient fusion beats. A control unit 15 is connected tothe determination unit 13 and to the pulse generator 2 for controllingthe pulse generator 2 to deliver stimulation pulses at a controlled AVinterval which is shorter than the incipient fusion AV-interval.

The atrial filter and measurement unit 17 and the evoked responsedetector 4 are disconnected by switches 19 and 11 from their respectiveleads 5, 6 during stimulation.

The evoked response detector 4 has filter and measuring unit 10. Thefiltered ventricular signals picked up by the ventricular electrode 7are supplied to a storage unit 21, an averaging unit 16 and to acomparator 12 for detecting incipient fusion by comparing the averageamplitude obtained during a predetermined time window of the cardiaccycle from the averaging unit 16 with suitably selected limit values. Asfollows from the above mentioned publication by Åsa Uhrenius et al.completely stimulated captures result in a comparatively large constantaverage amplitude whereas an incipient fusion results in a decrease ofthe absolute value of this average amplitude.

As an alternative, the averaging unit 16 can be adapted to form arunning average value of the measured ventricular signals during thepredetermined time window from a predetermined number of the latestcardiac cycles and the comparator 12 can be adapted to receive therunning average value and compare the average value obtained during thetime window of each cardiac cycle with the running average value fromimmediately preceding cardiac cycles.

The above mentioned limit values of the comparator 12 can be selectedsuch that e.g. a 10% decrease of the measured average amplitude comparedto the average amplitude in a situation of completely stimulated captureis indicated as an incipient fusion. Thus, a decrease of the absolutevalue of the average amplitude from e.g. 26 mV to e.g. 23, 5 mV can beinterpreted as incipient fusion. In this case, a running-average valueas described above of e.g. the ten last cardiac cycles, is suitably usedas limit value in the comparator 12 for obtaining an acceptablesignal-to-noise ratio.

A timer 14 is provided for determining the evoked response time windowduring which the ventricular signal is measured and stored. This evokedresponse window normally extends from 15 to 55 msec after stimulation.

Thus, after a blanking time of about 15 msec the measured evokedventricular signal is sampled and digitized during this evoked responsetime window and the average value of these samples is formed. Thisprocedure is performed in the averaging unit 16, which thus supplies tothe comparator 12 an average amplitude value obtained during the timewindow for each heart beat. A suitable sampling frequency can be e.g.512 Hz, which results in about 20 samples per beat.

As also follows from the publication by Åsa Uhrenius et al., thevariation in the average amplitude from different cardiac cycles iscomparatively small in a situation of completely stimulated capture,whereas this variation increases in a fusion situation. Thus, as analternative embodiment, the comparator 12 can be adapted to compare thevariability of average amplitude values obtained from different cardiaccycles with a predetermined variability limit to detect an incipientfusion.

The variability criterion for indicating incipient fusion normallyshould be more strict than the above discussed amplitude criterion.Thus, a variability increase in the average amplitude values of e.g. 25%compared to the variability at completely stimulated capture can be usedas variability criterion in the comparator 12 for indicating incipientfusion. An increase of the peak to peak variability in the averageamplitude values from different cardiac cycles from e.g. 2.5 mV to e.g.3.0 mV can be interpreted as incipient fusion. Also is this case arunning average value from e.g. the ten latest cardiac cycles shouldpreferably be used.

As a further version of this embodiment, the control unit 15 can beadapted to carefully modulate the AV-interval with e.g. ±5 msec or ±10msec. A large variability appearing in the average amplitudes is then areliable indication of fusion.

As still another alternative, the control unit 15 can be adapted toprolong at regular intervals the AV-interval with a predeterminedamount, e.g. 10 msec, and the average amplitude or variability criteriadescribed above are checked. If the average amplitude or variabilitycriteria then, for this prolonged AV-interval, indicate fusion orincipient fusion, the AV-interval is shortened by 20 msec. If no changesin average amplitude or variability are noted, the AV-interval is thecorrect one. This would mean that the heart stimulator chooses anAV-interval which is approximately 20 msec shorter than the AV-intervalat which incipient fusion is detected. In this way, a type of check isperformed to determine if the heart stimulator operates close to fusion,and in this way incipient fusion can be detected at a very early stage.

In the heart stimulator according to the invention it is also possibleto utilize both above described amplitude and variability criteria fordetermining an incipient fusion which normally further improves thedetection reliability.

To obtain a reliable result it is also desirable to eliminate any DClevel in the measured ventricular signal. This can be performed bysampling the measured ventricular signal before the emission of astimulation pulse and forming an average value of these samples during acardiac cycle. This average value represents the DC level and issubtracted from each sample of the subsequent measured ventricularsignal.

FIG. 2 shows in greater detail one embodiment of the evoked responsedetector of the heart stimulator according to the invention. Theventricular signal picked up by the lead 5 with its electrode 7 in FIG.1 is supplied to a highpass filter 20. An amplifier 22 and an A/Dconverter 24 are provided for amplifying and A/D converting respectivelythe filtered signal. A digital signal processor 26 calculates theaverage amplitudes of the measured ventricular signals and compares themwith suitably selected limit values as described above for detecting anincipient fusion.

FIG. 3 is a flow chart illustrating the function of the embodimentillustrated in FIGS. 1 and 2 of the heart stimulator according to theinvention for achieving 100% paced ventricular capture while optimizingthe AV-interval with respect to hemodynamic conditions. In step 40 an AVinterval is selected which is optimal with respect to the ventricularfilling of the patient in question. This AV interval is programmed by adoctor. The pulse generator 2 starts the HOCM therapy mode with a shortAV-interval, step 42.

The evoked response signal average amplitude during each heart beat ismonitored by the evoked response detector 4, and the AV interval isprolonged if it is shorter than the programmed AV interval, step 44.

It is checked that the evoked response average amplitude has asufficiently high, substantially constant absolute value, in step 46. Ifso, the AV-interval is prolonged while monitoring the evoked responseamplitude according to step 44. If not, the AV-interval is shortenedwith e.g. 5 msec while monitoring the evoked response amplitude, at step48.

It is then checked whether the average value formed from sampled valuesof the evoked response signal as described above maintains asufficiently high and constant absolute value according to thepredetermined requirements, in step 50. If so, the procedure reverts tostep 44, viz. the AV-interval is once again prolonged, provided that itis shorter than the programmed AV-interval, while monitoring the evokedresponse amplitude, and the procedure is continued to step 46 asdescribed above. If not, the procedure reverts to step 48, viz, theAV-interval is further shortened with 5 msec while monitoring the evokedresponse amplitude.

Thus the heart stimulator according to the invention is operating at anAV-interval which is as close as possible to the optimal AV-intervalprogrammed by a doctor while securing all the time that occurrence offusion is avoided. In this way a continuous suboptimization is obtainedof the programmed optimal AV-interval set by the doctor. If the evokedresponse average amplitude does not satisfy predetermined criteria withrespect to the absolute averaged value of the amplitude and possibly thevariability of the amplitude, the AV-interval is automatically shorteneduntil these criteria are again satisfied.

FIG. 4 is a block diagram of the basic components of a second embodimentof the heart stimulator according to the invention.

This embodiment has, in addition to the elements of the embodiment shownin FIG. 1, a respiration signal determining unit 28, which is suppliedwith the average signal values generated by the averaging unit 16. Therespiration signal determining unit 28 generates a respiration signal,representing the respiration rate of the patient, from a predeterminednumber of evoked response average values. The respiration signal issupplied to the AV-interval determining unit for use in the control ofthe pulse generator 2. The use of the respiration rate to control theoperation of a pacemaker, is well known to the person skilled in theart, cf. e.g. U.S. Pat. No. 4,702,253, and is therefore not describedherein.

Although modifications and changes may be suggested by those skilled inthe art, it is the intention of the inventors to embody within thepatent warranted hereon all changes and modifications as reasonably andproperly come within the scope of their contribution to the art.

1. A heart stimulator comprising: a stimulation pulse generator adaptedfor interaction with an atrium and a ventricle of a heart for generatingatrial stimulation pulses and ventricular stimulation pulses having anassociated AV-interval; an atrial sensor, adapted for interaction withsaid atrium, for sensing atrial signals therefrom; an evoked responsedetector adapted for interaction with said ventricle for receivingventricular signals, for detecting evoked response signals and includingan averaging unit which forms an average amplitude value of respectiveevoked response signals during a predetermined time window of eachcardiac cycle, said average amplitude values exhibiting a variabilityfrom cardiac cycle to cardiac cycle, and a comparator which comparessaid average amplitude value for each cardiac cycle with a predeterminedvariability limit, to generate a comparison result; a determination unitconnected to said evoked response detector and to said atrial sensor,which determines an incipient fusion AV-interval to be present if saidcomparison result indicates said variability limit was exceeded; and acontrol unit connected to said determination unit and to said pulsegenerator for controlling timing of said ventricular pulses to produce acontrolled AV-interval which is shorter than said incipient fusionAV-interval to maintain 100% stimulated beating of said heart.
 2. Aheart stimulator as claimed in claim 1 wherein said control unitmodulates said AV-interval by a predetermined amount, to obtain amodulated AV-interval, and wherein said comparator compares respectiveaverage amplitude values for respective cardiac cycles wherein themodulated AV-interval was in effect, to said predetermined variabilitylimit value.
 3. A heart stimulator as claimed in claim 2 wherein saidcontrol unit regularly prolongs said AV-interval by said predeterminedamount, to obtain said modulated AV-interval.
 4. A heart stimulator asclaimed in claim 1 wherein said evoked response detector samples anddigitizes said evoked response signals for each heartbeat in apredetermined evoked response time window beginning at a predeterminedtime after delivery of a stimulation pulse from said pulse generator tosaid ventricle, thereby obtaining sampled amplitude values, and whereinsaid averaging unit forms said average amplitude value from said sampledamplitude values.
 5. A heart stimulator as claimed in claim 4 whereinsaid evoked response detector samples and digitizes said evoked responsesignals with a sampling frequency and with a length of said evokedresponse time window so that approximately 20 of said sampled amplitudevalues are obtained within each evoked response time window.
 6. A heartstimulator as claimed in claim 5 wherein said evoked response detectordetermines a DC level of said ventricular signals and subtracts said DClevel from sampled amplitude value, before said averaging unit formssaid average amplitude value.
 7. A heart stimulator as claimed in claim1 further comprising a respiration signal determining unit whichdetermines a respiration signal associated with said patient,representing a respiration rate, from a predetermined number of saidaverage amplitude values, and supplies said respiration signal to saiddetermination unit, and wherein said determination unit generates saiddetermination result dependent on said respiration signal.
 8. A heartstimulator as claimed in claim 7 wherein said respiration signaldetermination unit determines said respiration signal from variationsamong a predetermined number of said average amplitude values.